Taking a Bite Out of Prey

A Glowing Parasitic Shark with a Unique Mark

Of course, with only twelve weeks to cover bioluminescence, we’ve only begun to scratch the surface of the varied groups that exhibit it. Our final story of this season covers another fish species. This one uses its glow as a lure, like the anglerfish, but with a slight twist. For one thing, this species uses its lure to not just attract prey, but also parasitic victims. The cookiecutter shark, Isistius brasiliensis, is a small (42–56cm long) cartilaginous fish. While it can consume some smaller prey items, it is most well known for gouging out round pieces of flesh from larger organisms to consume. This gives the appearance of being cut out with a cookie cutter, hence the name.

While it can consume smaller squid whole, the shark’s favorite targets for its parasitism are larger fish and marine mammals, like the elephant seal seen above. In order to mask its appearance, the underside of its body is covered in luminescent organs, similar to the firefly squid. How can this be used as a lure? This fish possesses a dark collar around the area of its gill slits where no bioluminescence is produced.

This dark collar mimics the silhouette of a small fish from below, while the counter-illumination of the bioluminescence obscures the rest of the shark. While the use of this collar as a lure has yet to be fully confirmed, it provides a fascinating contrast to the anglerfish discussed earlier this season. Instead of using the light it produces as a lure, it would actually be the absence of light that draws in prey. Cookiecutter sharks are also known to travel in schools, which would make the lures even more effective and realistic.

Once a potential prey item comes in close, the shark grabs it with its jaw and creates a strong suction force. Its upper teeth anchor it into the victim and the lower teeth slice into the prey as the shark twists to create a circular cut averaging 5cm in diameter and 7cm deep. Attacks on humans are rare, mainly because there is little overlap in the shark’s habitat (it favors offshore waters) and human activity, but for some marine mammals, cookiecutter shark bites are ubiquitous where the latter are found.

Cookiecutter sharks are not particularly picky about what they bite into, either. In fact, a number of submarines suffered damage to the neoprene boots on their sonar domes in the 1970s. This was initially thought to be some kind of enemy weapon, until a cookiecutter shark was discovered still attached to one. This species has also caused damage to underwater cables. The solution to such nuisance attacks has been fitting the targeted areas with fiberglass.

Other than its somewhat unorthodox feeding method, the cookiecutter shark behaves similarly to many other bioluminescent marine organisms. It migrates up and down in the water column on a day/night cycle (diving during the day and rising at night). Its overall effect on the species it parasitizes is still not fully known, though the most susceptible individuals are those that are already weakened or diseased.

I hope you all enjoyed Season 8 of Nature Stories. It was a fascinating journey throughout a variety of habitats looking at different organisms that glow. The parallels between many of them are quite striking and I feel that it is an excellent example of convergent evolution: an adaptive trait that has developed multiple times across different lineages giving rise to similar appearance or behavior. As always, please like and share this with others you think might enjoy it and let me know if there is a particular topic you would like me to cover for a future season.